Step 1Position an oven rack 6 inches from the broiler element. Heat the oven to broil, on a medium setting, if possible, and turn on the oven light. Put the cereal in a large bowl.
Step 2Scatter the butter in a 13- by 9-inch cake pan and place in the oven to cook until the butter melts and its milk solids turn light golden brown, 3 to 5 minutes. Watch it carefully. Immediately remove the pan from the oven and carefully tilt it to evenly coat the bottom with the butter. Quickly spread the marshmallows in a single layer in the pan.
Step 3Return the pan to the oven rack and broil, watching the progress the whole time, until the tops of the marshmallows are very dark golden brown and the marshmallows are puffed, 2 to 4 minutes. Carefully and immediately slide the marshmallows out of the pan and into the bowl with the cereal. Leave behind any butter slicking the pan; don’t scrape anything out.
Step 4Carefully fold the marshmallow mixture together until the cereal is evenly coated. Scrape the mixture into the pan and press it in an even layer. Cool completely, then cut into 24 bars to serve.
After pressing the marshmallow treats into the pan, arrange 24 squares of milk chocolate in a 6-by-4 grid on top. Return to the oven and heat just until the chocolate has softened, about 1 minute. Cut 6 graham cracker squares into 4 smaller squares each. Press each square into a piece of chocolate.
Big Batch Campfire Marshmallow Bars
Follow the method above using a half sheet pan (17 by 11 inches) and 9 ½ cups crisped rice cereal, 6 ½ tablespoons salted butter and one 16-ounce bag of marshmallows.